Nick,
if you can have access to the car being offered to you, consider checking the data contained in VIN with realities, i.e.
- check if the VIN the same on the plate as it is on some parts where the last three digits should be marked up as well (this is well explained in our Tech Manual and on the very instructive webpage
http://www.motoringinvestments.com/280SL_BuyersGuide.htm- check whether the parts specific to 1970 model are there (again check the modification schedule for W113 as available in certain books or in our Tech Manual)
The Datacard would be obviously better as it tells more specifically about certain options etc, but the above should be a good start.
BTW, the datacard should be available to you through your coal MB dealer (I got mine this way), but they request the proof of ownership of the car. I am therefore quite surprised you were able to request yours at the MB Germany.